The present invention relates to a two-cycle automotive engine with a direct fuel injection system, and more particularly to a system for controlling engine speed to a predetermined speed at an idling state of the engine.
In the two-cycle engine, one cycle of the operation in one cylinder is completed within one revolution of a crankshaft. Accordingly, sufficient amount of air is not induced into the cylinder.
In a crank-case-scavenged two-cycle engine, vacuum is produced in an enclosed crankcase of the engine to induce fresh air when a piston ascends. When the piston descends, burned gas is discharged through an exhaust port while the fresh air in the crankcase is admitted into a cylinder, thereby scavenging the cylinder. On the other hand, a throttle valve is provided in an intake system for controlling engine power. Quantity of intake air is adjusted in accordance with an opening degree of the throttle valve.
However, at light engine load, since the intake air quantity is small, the scavenging becomes insufficient, resulting in misfire and hence in an irregular engine operation. In addition, torque characteristics corresponding to engine operating conditions becomes uneven. Moreover, at heavy engine load, volume of the crankcase is insufficient for scavenging. Since combustion in an idling state is unstable, it is difficult to keep engine speed constant in the idling state and to sufficiently increase output torque at the heavy engine load.
Japanese Patent Application Laid-Open 57-86522 discloses a system in which a return passage is provided between a cylinder and a crankcase, separated from a scavenge passage. During a compression stroke, an amount of air-fuel mixture in the cylinder is returned to the crankcase through the return passage, thereby reducing an effective compression stroke of a piston.
Japanese Patent Application Laid-Open 56-113020 discloses an electronic governor for a diesel engine in which a difference between a desired engine speed and an actual engine speed is controlled by controlling quantity of fuel supplied to cylinders so as to maintain the difference within a predetermined small range.
In the former conventional system, the mixture extracted from the cylinder affects a compression ratio and an explosion pressure so that a desired engine power is not obtained. Further, since the pressure in the crankcase is increased, fresh air can not be sufficiently induced in the crankcase, which results in reducing intake air into the cylinder.
In the latter conventional system, the quantity of the fuel is controlled by changing a duty ratio of pulses applied to a solenoid operated control valve. A range of the quantity of the fuel controlled by the control valve is small. Accordingly, the system can be employed for an engine in which variation of the engine speed is comparatively small. Thus, if the system is employed in the two-cycle engine, idling speed of which is unstable because of unstable combustion, it is necessary to cause the stratified combustion in order to reduce fluctuation of the idling speed.
However, in the engine operated by the stratified combustion, even if the quantity of air is controlled, the engine speed at idling state does not change. Therefore, an idle speed control valve for controlling the engine speed can not be provided in a bypass formed in the intake pipe around a throttle valve.